“It is important to send a clear message to North Korea that provocative actions have consequences,” Mrs. Clinton said after meeting in Tokyo with the Japanese foreign minister, Katsuya Okada. “We cannot allow this attack on South Korea to go unanswered by the international community.”

The comments were the first from Mrs. Clinton on what South Korea has called a deliberate North Korean torpedo attack, which killed 46 sailors. Mrs. Clinton declined to lay out the options for a response, saying that would be “premature.”

But other administration officials said they included action by the United Nations Security Council and joint American-South Korean naval exercises that could include antisubmarine warfare patrols. South Korea is also likely to squeeze the North economically by reducing trade ties.

Mrs. Clinton, who flew to Shanghai later on Friday, will consult with Chinese officials in Shanghai and Beijing on the matter. China’s support is considered crucial, both because it holds a veto in the Security Council and because it is North Korea’s neighbor and the principal supporter of the North’s leader, Kim Jong-il.

So far, China has been reluctant to condemn North Korea for the torpedo attack on the 1,200-ton corvette, the Cheonan — a reticence that has outraged many in South Korea.

Photo

Secretary of State Hillary Rodham Clinton met with the Japanese foreign minister Katsuya Okada in Tokyo on Friday. Credit
Pool photo by Itsuo Inouye

The escalating tensions between North and South Korea have scrambled what had been planned as three days of economic and security talks between China and the United States in Beijing next week. Now, those discussions will be partly eclipsed by what a senior American official characterized as the greatest military provocation on the Korean Peninsula in decades.

Already, there is evidence that the tensions have influenced security calculations in the region. After declining for months to work out a deal to move a Marine Corps air base on the island of Okinawa, Japan now appears to be working toward a resolution, driven in part by a reaffirmation of the role the United States military plays in Japan’s security, officials said.

Mr. Okada said his government was trying to convince residents of Okinawa that “in the current security environment, the presence of U.S. forces is indispensable for the security of Japan.” He said he hoped a deal would be reached by the end of May, the government’s self-imposed deadline.

China is likely to be a tougher sell. President Hu Jintao hosted Mr. Kim during his train trip to China this month, just days after meeting with South Korea’s president, Lee Myung-bak. That showed China’s instinct not to take sides in disputes between the North and South.

Still, Mrs. Clinton began laying out the case for China’s support in Tokyo, noting that the South Korean government’s investigation of the sinking had been a “thorough and comprehensive scientific examination,” echoing the words Chinese leaders had used in calling for a scientific investigation.

The evidence collected, including part of a torpedo propeller, was “overwhelming and condemning,” she said.

“Let me be clear: this will not, and cannot, be business as usual,” Mrs. Clinton said, speaking in a grave tone. “There must be an international, not just a regional, but an international response.”

Photo

Secretary of State Hillary Rodham Clinton arriving for a meeting with the Japanese foreign minister Katsuya Okada in Tokyo on Friday.Credit
Pool photo by Saul Loeb

While Mrs. Clinton tours the American pavilion on the vast fairgrounds of the Shanghai Expo this weekend — the main event during her stop here — her aides will be huddling elsewhere, making the case that China must back punitive measures against the North.

“We’d like to see them acknowledge the reality of what happened and then join with South Korea, Japan and us in fashioning a response,” said a senior administration official, speaking on the condition of anonymity because he was not authorized to speak publicly.

The administration finds itself in a delicate spot: orchestrating international support for South Korea while also deferring to President Lee on the timing and nature of the response. The United States is wary of aggravating an already tense situation; North Korea, which denies the torpedo attack, threatened to respond to military retaliation with “all-out war.”

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Mrs. Clinton did not speak to reporters traveling on her plane, breaking a common practice on trips by the secretary of state. An aide said she spoke to President Obama about the crisis while en route.

Despite the bellicose language from the North, American officials said they did not believe the country was trying to precipitate a military attack.

“The hope is that this was a one-off action,” one official said. “We have not seen evidence that this is the first step on the road to war.”

When Mrs. Clinton wraps up her meetings in China next week, she will stop in Seoul, South Korea’s capital, to give public backing to President Lee. By then, aides said, there will be clarity on what steps he plans to take.

A version of this article appears in print on May 22, 2010, on Page A4 of the New York edition with the headline: Clinton Issues Warning to North Korea Over the Sinking of a South Korean Vessel. Order Reprints|Today's Paper|Subscribe